The present invention relates to an Ni-Cr-W heat-resistant alloy which has a high creep rupture strength and superior high temperature fatigue strength and which exhibits a good workability and, hence, can easily be worked into rods, wires, plates tubes and so forth. The invention relates also to a method of producing the above-mentioned Ni-Cr-W alloy.
The present inventors have already developed an alloy containing, as the main components, 23%Cr-18%W-Ni and exhibiting a high workability, as well as a high creep rupture strength. This alloy is disclosed in Japanese Patent Publication No. 33212/1979. Hitherto, a mere solution heat treatment at 1250.degree. to 1350.degree. C. has been adopted as a method of heat-treating this alloy. The heat-treated alloy has a structure which essentially consists of simple austenite grains of grain size larger than 100 .mu.m, except small amount of undissolved precipitates.
It has been proved that the alloy having such structure has a disadvantage in that its high temperature fatigue strength is relatively low, although its creep rupture strength is sufficiently high. Besides the creep rupture strength, the high temperature fatigue strength is an important property which rules and restricts the design of high-temperature equipments such as heat exchangers. As a matter of fact, however, the high temperature fatigue strength is incompatible with the creep rupture strength from a metallurgical point of view. Namely, if one of the high temperature fatigue strength and creep rupture strength is increased preferentially, the other is decreased undesirably. For instance, in the case of the 23%Cr-18%W-Ni alloy disclosed in Japanese Patent Publication No. 33212/1979 mentioned before, the grain is refined to provide an improved high-temperature fatigue strength if the solution heat treatment temperature is lowered to below 1150.degree. C., but this is accompanied by a reduction in creep rupture strength which is one of the important properties of this alloy.